1. Field of the Invention
This invention relates to the conversion of heavy petroleum feedstocks and more particularly to processes for coking residual petroleum feedstocks in the presence of free radical inhibitors.
2. Description of the Prior Art
Coking is an increasingly important processing area in petroleum refining. As high quality crudes become scarcer and more expensive, refineries must process increasing quantities of lower quality crudes which contain or, upon processing, form large amounts of high-boiling materials that are typically treated in coking units. Thus, the quality and quantity of products produced by coking processes can have a large impact on overall refinery yields because the relative amount of feedstock to be coked generally increases as the quality of crude oil material decreases.
Principle heavy petroleum coking feedstocks are high-boiling virgin or cracked petroleum residua such as virgin reduced crude, bottoms from vacuum distillation (vacuum reduced crude), thermal tar and other residue and blends thereof. Coking enables efficient conversion of these less desirable petroleum fractions to more desirable distillate products and a byproduct coke.
A variety of coking methods are known in the art including delayed, fluid, and moving bed coking processes.
Delayed coking is a process wherein the feedstock is preheated to a coking temperature, generally between 800.degree. F. to about 1100.degree. F. and more usually between about 850.degree. F. to 950.degree. F. The preheated feedstock is then fed to the bottom of a delayed coker drum. The coking feed is allowed to soak in its own heat in the delayed coker at a low pressure, generally from about one atmosphere to about 10 atmospheres absolute, preferably from about three atmospheres to about seven atmospheres absolute. The cracked vapors are continuously removed overhead so as to recover the distillate fuels while coke is allowed to build up in the drum to successively higher levels. When the drum is filled with coke, the preheated feed is diverted to a succeeding drum and the former drum is steamed out and cooled. The coke is then removed from the cooled drum.
Fluid coking is a process wherein feedstock is sprayed into a bed of hot fluidized coke particles in a reactor. The feedstock is cracked into lighter vapor-phase products and into coke, the coke being deposited on the particles of the fluidized bed. The particles of coke are circulated from the reactor to a burner wherein they are partially combusted with an oxygen-containing gas in a moving, fluid, or transfer line combustion zone and thereby raised in temperature, some of the heated coke particles being returned to the reactor for further use, the remainder of the coke being withdrawn as a byproduct. In a typical fluid coking unit the feedstock is converted to about 70% of normally liquid products and about 25% of coke, and 7-8% of the latter (based on charge) is consumed in the burner to provide heat for the process.
Moving bed coking is a process wherein the feedstock is uniformly distributed to the top of a mass of heated granular petroleum coke particles maintained in a reactor through which the particles downwardly pass by gravity. The liquid hydrocarbon charge is converted by the heat of the particles to produce lower-boiling vapors and a dry coke coating on the particles. The coated coke particles are withdrawn from the bottom of the reactor and either recovered as a coke byproduct or passed to a burner similar to that employed in fluid coking processes to raise the coke particle temperature for return to the coking reactor.
Condensation and thermal cracking are two major reactions which take place in the coking process. The thermal cracking results in bond-breaking and produces lighter molecules (distillates and gases). The condensation is an undesirable reaction because it produces a low value product, i.e., coke. The coke formation is believed to proceed through free radical condensation wherein the radicals are initially formed by thermal dissociation (Equation 1). ##STR1## Several reactions may take place for the free radical. It may combine with hydrogen to form the stable, lighter molecule as shown in Equation 2. It also can be dehydrogenated to form an olefin (Equation 3). Moreover, it can condense with aromatic hydrocarbons to form heavier molecules (Equation 4). The condensation can be repeated forming coke (Equation 5). ##STR2##
The principle charging stocks for coking operations are high boiling virgin or cracked petroleum residues which may or may not be suitable as heavy fuel oils. An important use of coke is as domestic or industrial fuel although a substantial tonnage is processed and used in making carbon or graphite electrodes for use in the metals industries. However, the dynamic manner in which fluid coke is formed yields a solid product having physical properties which make it undesirable for this latter application. Delayed coking, on the other hand, when processing a sufficiently aromatic feedstock, can provide a premium quality coke product.
A primary objective of all of the various known coking processes has been to convert as large a proportion as possible of the feedstock to lighter hydrocarbon fractions while keeping coke formation to a minimum. The coker feedstock is completely converted to lighter and heavier materials. The lighter products (resulting from cracking) are gas, some gasoline, and gas oil. The heavier product (resulting from condensation reactions) is coke. The various product yields are affected by the coking tendency of the charge stock (e.g., as indicated by the Conradson Carbon Residue), by the process employed (delayed or fluid) and by the process conditions. The yield of distillates is maximized by coking at low pressures. At higher pressures more gas and coke are produced, and the liquid product contains more gasoline. The yields of gas and gasoline also increase with increasing temperatures; the yield of gas oil decreases. Moreover, the research octane number of the gasoline increases linearly with temperature: for example, from 72 at 930.degree. F. to 87 at 1057.degree. F. Gasolines produced at higher temperatures are unstable and require finishing operations such as clay treating or mild hydrogenation. The gases produced at higher temperatures are olefinic: at an average temperature of 955.degree. F. they are 50% olefinic, as compared with 15% at temperatures of about 850.degree. F.
Present delayed coker reactors must be operated within a relatively narrow range of conditions which limits the degree of control over product yield distribution and over product qualities. As noted above, a principle limitation of delayed cokers is the furnace outlet temperature which in turn limits the temperatures in the delayed coking drums. This limitation is of relatively minor importance in plants where the more valuable gaseous and liquid products produced by delayed coking are a relatively small percentage of the total volume of similar products produced in the complete refinery. However, improved product flexibility would be a considerable asset to the process and is particularly important in refineries processing heavy crudes such that the coker products have a major influence on overall refinery yields. Inasmuch as high quality crudes are becoming increasingly scarce and expensive, the processing of heavy crudes is becoming increasingly important today.
The literature is replete with various means employed to decrease the formation of coke, carbonaceous deposits and other contaminants in a wide variety of hydrocarbon processes. For the purpose of illustrating the prior art, the following patents are considered exemplary.
U.S. Pat. No. 3,342,723 discloses a method of inhibiting the formation of coke-like deposits in oil refining apparatus by the addition of various antifouling agents to a hydrocarbon liquid. Typical antifouling agents are aromatic compounds such as hydroquinone, orthophenylene diamine, and catechol. The antifouling agents are employed in the treatment of any component of petroleum which is exposed to high temperatures.
In U.S. Pat. No. 3,654,129, a polymerization inhibitor is added to a coke-forming hydrocarbon charge stock to decrease coke formation and increase catalyst life. The inhibitor is selected from the group consisting of phenols, aromatic amines and thiophenols.
U.S. Pat. No. 3,772,182 discloses a process for inhibiting fouling in petroleum refining and chemical processing equipment by means of an antifouling composition which contains a diarylamine compound such as diphenylamine.
Although the suppression of coke is considered desirable for one or more reasons, e.g., to extend catalyst life, prevent heat transfer loss due to the formation of high temperature deposits on metal surfaces and/or otherwise increase the yield by minimizing the loss represented by deposition of coke and other carbonaceous material, the prior art does not suggest deliberately inhibiting the formation of coke in a hydrocarbon process designed to yield a coke product such as a delayed, fluid or moving bed coking process.